1. Field of the Invention
This invention relates to a method and apparatus for matching positions of images. This invention particularly relates to an improvement in position matching of two images of a single same object.
2. Description of the Related Art
Techniques for comparing at least two images of a single same object, finding a difference between the images, and making an examination of the object, or the like, in accordance with the difference between the images have heretofore been conducted in various fields.
For example, in the fields of production of industrial products, there have heretofore been conducted techniques for comparing an image of a certain product, which image has been recorded when the product is in a state of a new article, and an image of the product, which image has been recorded after a durability test has been made on the product, paying attention to part of the product, at which the difference between the images is large, and determining part of the product, at which the durability is to be enhanced. Also, in the medical fields, there have heretofore been conducted techniques for comparing a plurality of radiation images of a certain diseased part of a patient, which radiation images have been recorded in a time series mode, with one another, finding the condition of advance of the disease or the condition of cure of the disease, and determining a plan for treatment of the disease.
As described above, the techniques for comparing at least two images are conducted ordinarily in various fields. For making the comparison of the at least two images, the at least two images are often displayed on an image display device, and the like. Specifically, the images having been recorded are transformed into image density signals or luminance signals, and visible images are reproduced from the image density signals or luminance signals and displayed on the image display device, and the like. Alternatively, visible images are reproduced on a medium, such as film, by the utilization of a printer.
In cases where at least two images, which are to be compared with each other, are outputted, the images are ordinarily outputted so as to stand side by side with each other. In cases where the comparison is made, a matter of primary concern for the person, who sees the outputted images, is the difference between the images. However, it is not easy to find the difference between the images, which are merely located so as to stand side by side with each other. Particularly when the difference is small, it is difficult to find the difference in the manner described above. Therefore, there is a strong demand for enhancement of performance of the comparison.
Accordingly, ordinarily, techniques for performing inter-image operation processing, such as a subtraction process, on corresponding pixels in the at least two images to be compared with each other, and thereby extracting and enhancing the difference between the images are conducted. (One of such techniques is proposed in, for example, Japanese Patent Application No. 11(1999)-342900.) In cases where only the difference between the images is thus extracted and enhanced, the difference between the images is capable of being reliably perceived by the person, who sees the outputted image. As a result, the diseased part, which is advancing or curing, is capable of being found reliably.
In cases where the inter-image operation processing is to be performed, it is necessary that positions of the two images, which are to be subjected to the comparison, i.e. the inter-image operation processing, be accurately matched with each other. If the positions of the two images are not accurately matched with each other, an artifact of structures embedded in the images will occur from the inter-image operation processing, which is performed on the corresponding pixels in the images.
Besides the images having been recorded successively in the time series mode, two images, which have been acquired at two different points of time apart from each other, are often subjected to the comparison. In such cases, ordinarily, a shift occurs between the two acquired images due to a deviation in orientation of the object during the image recording operation, error, and the like. Therefore, before the inter-image operation processing is performed, it is necessary for the shift to be compensated for.
Accordingly, approximate position matching processing is performed on the entire area of at least either one of the two images of the same object, which images are to be compared with each other. As the approximate position matching processing, global position matching processing (a linear transform, e.g. an affine transform), such as rotating processing, parallel translation, and image size enlargement or reduction processing, may be performed. Alternatively, as the approximate position matching processing, local position matching processing with nonlinear strain transform processing (i.e., warping) in accordance with a corresponding position relationship, which has been obtained from template matching for each of local area limited regions, may be performed. The nonlinear strain transform processing may be performed with, for example, curve fitting utilizing two-dimensional polynomials. As another alternative, as the approximate position matching processing, a combination of the global position matching processing and the local position matching processing may be performed. (Techniques for performing the approximate position matching processing are described in, for example, Japanese Unexamined Patent Publication Nos. 7(1995)-37074and 8(1996)-335721.) With the approximate position matching processing described above, position matching is capable of being effected to a certain extent with respect to the entire areas of the images.
However, for example, in cases where image patterns of similar textures are embedded in adjacent local area limited regions as in the cases of rib image patterns in chest radiation images, the approximate position matching processing described above has the problems in that a shift often remains between certain local area limited regions.